It is definitely data related since it is only one of 3 companies.  Identifying which file is trickier - can you open anything on the AR or SO menu?

Then I suspect the BKARINVT file which is receivables invoices.  Make a backup copy and try reindexing it.

Try BKART.  Also see if you can open those files using Maintain Database and go to the beginning and end of the file without errors or obvious corruption.

For future reference, the problem turned out to be a Pervasive extension file of the MKECLASS.B01 file that was created when the server ran low on disk space.  The Pervasive extension files have an extension of .^01 rather than the original .BXX extension and are also the type of files that get created when an individual file exceeds 2 GB.  In this case, once space on the server was freed up, restoring a backup of MKECLASS.B01 and deleting the MKECLASS.^01 resolved the problem.